1. Field of the Invention
The present invention relates to liquid crystal display devices and, more particularly, to a liquid crystal display apparatus suitable for projecting a graphic pattern or the like, written on a liquid crystal cell, onto a screen by using a liquid crystal light emitter actuated by the heat from a laser beam.
2. A Description of the Prior Art
As conventional liquid crystal display apparatus based on a liquid crystal light emitter actuated by the heat from a laser beam, there is known a projection type display or the like in which a laser beam is irradiated on a liquid crystal cell formed of a liquid crystal of smectic phase and an image written in the liquid crystal cell is projected onto a screen by utilizing a thermoelectric optical effect.
FIG. 1 of the accompanying drawings shows a structure of an optical system of the above conventional projection type liquid crystal display apparatus based on a liquid crystal light emitter actuated by the heat from a laser beam.
As shown in FIG. 1, a laser beam emitted from a laser diode 30 disposed within a laser block 1 is traveled through galvanoscanner mirrors 3a, 3b, which deflect a laser beam on a liquid crystal cell 7 (described later on) in the Y-axis direction and X-axis direction, a relay lens 2, a first condenser lens 4, and then reflected on a dichroic mirror 5 to become a writing laser beam 6. This writing laser beam 6 is focused on a liquid crystal surface of the liquid crystal cell 7. A point at which the writing laser beam 6 radiates the liquid crystal cell 7 is held in this state.
The writing laser beam 6 irradiated on the surface of the liquid crystal cell 7 is scanned by the two Y-axis and X-axis galvanoscanner mirrors 3a, 3b to thereby draw a graphic pattern or the like on an arbitrary position on the liquid crystal surface of the liquid crystal cell 7.
The graphic pattern or the like drawn on the liquid crystal surface of the liquid crystal cell 7 is projected on to a screen 9 by means of a projection light 10 from a projection lamp 8. That is, the projection light 10 from the projection lamp 8 is converged by a condenser lens 11 and irradiated on the the liquid crystal cell 7 from its rear side through the dichroic mirror 5, whereby the graphic pattern or the like on the liquid crystal surface of the liquid crystal cell 7 is projected on to the screen 9 via a projection lens 12.
A theory in which the graphic pattern or the like is drawn on the surface of the liquid crystal cell 7 by the heat from the laser beam as described above will be described with reference to FIGS. 2A through 2D of the accompanying drawings. As illustrated, the liquid crystal cell 7 comprises two glass substrates 14 and 15. Transparent electrode layers 16, 17 made of indium oxide-tin oxide (ITO) or the like and insulating layers 18, 19 made of silicon dioxide (SiO.sub.2) or the like are respectively coated on the glass substrates 14, 15. A laser mirror 20 is formed on the glass substrate 14 which is located on the opposite side of the glass substrate 15 into which the writing laser beam 6 is introduced. The laser mirror 20 is formed of a dielectric optical multi-layer.
A space or vacant cell between the two glass substrates 14 and 15 is sealed by a sealing material 21 such as a thermosetting resin or the like.
A smectic liquid crystal 24 or the like is filled into the vacant cell sealed by the sealing material 21. The smectic liquid crystal 24 may be formed of a liquid crystal whose phase is changed in the order of a smectic A phase, a nematic phase and an isotropic phase.
If the liquid crystal 24 is returned in phase to the original smectic A phase after the phase transition from the smectic A phase to the isotropic phase occurred due to the heat from a laser beam, then a random orientation in the isotropic phase is retained in the smectic A phase and the light scattering state is formed, thereby retaining the memory state. If this memory state is erased, then the phase of the liquid crystal 24 is returned to the smectic A phase of the regular alignment by the application of a voltage.
The transparent electrode layers 16, 17 are formed in order to apply the voltage to the liquid crystal cell 7 when a displayed image is erased, while the insulating layers 18, 19 are formed in order to prevent the transparent electrode layers 16, 17 from being short-circuited by impurities doped into the liquid crystal 24. Further, the laser mirror 20 is coated on the glass substrate 14 in order to reflect the writing laser beam 6 introduced thereto from the laser diode 30 to thereby effectively utilize the energy of the laser beam.
A switch 22 and an AC power supply 23 are connected in series between the transparent electrode layers 16 and 17 of the above liquid crystal cell 7.
In the state shown in FIG. 2A, the switch 22 is in its off state and the liquid crystal 24 is in the smectic A phase of the regular alignment.
Then, when the writing laser beam 6 is introduced into the liquid crystal cell 7 from the glass substrate 15 side as shown in FIG. 2B, only a liquid crystal 24a irradiated with the writing laser beam 6 is changed in phase to the isotropic phase of the light scattering state and becomes a pixel 25 as shown in FIG. 2C so that a predetermined image may be drawn.
Then, when the switch 22 is turned on to apply a voltage across the transparent electrode layers 16, 17 from the AC power supply 23 as shown in FIG. 2D, the isotropic phase of the liquid crystal 24 is returned to the original smectic A phase of the regular alignment.
FIG. 3 of the accompanying drawings shows a structure of the optical system disposed within the above laser block 1 in the conventional liquid crystal display apparatus. In FIG. 3, like parts corresponding to those of FIG. 1 are marked with the same references and therefore need not be described in detail.
As shown in FIG. 3, a laser beam emitted from the laser diode 30 is introduced through a collimator lens 31 and an anamorphic prism 32 into a half-wave plate 33. A laser beam of a linear polarized P-component, for example, emitted from the laser diode 30 is collimated by the collimator lens 31. Then, the collimated laser beam from the collimator lens 31 is changed in spot shape from ellipse to circle by the anamorphic prism 32. A part of the laser beam (i.e., less than several percents) is changed by the half-wave plate 33 to a linear polarized S-component whose vibration direction is rotated 90 degrees.
The linear polarized P-component and S-component from the half-wave plate 33 are introduced into a polarizing beam splitter 34, in which they are separated into a traveling light and a reflected light. The S-component whose light amount is about 1 to 2% is reflected by the polarizing beam splitter 34 whereas other P-component is passed through the polarizing beam splitter 34.
The S-component reflected by the polarizing beam splitter 34 is converged by a collimator lens 35 that serves to monitor a laser beam and introduced into a photodiode 36 that is used to monitor the laser beam from the laser diode 30.
The laser beam passed through the polarizing beam splitter 34 is introduced into a quarter-wave plate 37. The linear polarized P-component is changed by the quarter-wave plate 37 into a circular polarized P-component and scanned by the galvanoscanner mirrors 3a, 3b shown in FIG. 1 so that it is focused on the liquid crystal 24 of the liquid crystal cell 7 as the writing laser beam 6, thereby writing a predetermined graphic pattern on the surface of the liquid crystal 24.
A position detecting reflected-back beam 39 that has been reflected by a reflecting layer (e.g., aluminum layer) 40 or the like disposed on predetermined positions (e.g., top and end of and right and left of X-axis and Y-axis directions) of the panel surface of the liquid crystal cell 7 is returned again into the laser block 1 by means of the galvanoscanner mirrors 3a, 33b. Of course, although the writing laser beam 6 is irradiated on the liquid crystal 24 of the liquid crystal cell 7 as the heat energy to change the liquid crystal cell 24 in phase from the smectic A phase to the isotropic phase thereby drawing a graphic pattern or the like, other extra energy is returned into the laser block 1 as the reflected-back beam 39.
The reflected-back beam 39 returned into the laser block 1 is changed again into the linear polarized S-component by the quarter-wave plate 37. The S-component is reflected by the polarizing beam splitter 34 and introduced into a collimator lens 41 that is used to detect a drawing position and a photodiode 42.
Further, when the surface of the liquid crystal cell 7 is scanned by the galvanoscanner mirrors 3a, 3b in the liquid crystal display apparatus based on a liquid crystal light emitter actuated by the heat from the laser beam, an automatic power control (APC) is effected such that an output power density of the laser diode 30 is made constant in response to the drawing speed.
FIG. 4 of the accompanying drawings shows in block form a conventional circuit for controlling a laser power density. In FIG. 4, like parts corresponding to those of FIGS. 1 and 3 are marked with the same references and therefore need not be described in detail.
As shown in FIG. 4, the laser block 1 that was earlier described with reference to in FIG. 3 includes therein the laser diode 30 serving as the laser beam source and the photodiode 36 that monitors the output of the laser diode 30. The output from the laser diode 30 which is detected by the photodiode 36 is fed through a current-to-voltage converter circuit 44 back to one input terminal of a comparator circuit 45 formed of an amplifier. Further, an output from the comparator circuit 45 is converted into a current by a voltage-to-current converter circuit 46 which is controlled by a pulse width modulation (PWM) wave from a PWM controller circuit 47 and then supplied to the laser diode 30 so that the output of the laser diode 30 is made constant.
The PWM controller circuit 47 and the comparator circuit 45 are both controlled by a computer (hereinafter referred to as a CPU (central processing unit)) 48 forming a system controller.
A reference voltage from a reference voltage setting circuit 52 is supplied to the other input terminal of the comparator circuit 45.
The reference voltage setting circuit 52 includes a first variable resistor VR.sub.1 that sets a reference voltage used to draw a bold line, a second variable resistor VR.sub.2 that sets a reference voltage used to draw a middle line and a third variable resistor VR.sub.3 that sets a reference voltage used to draw a fine line. The first to third variable resistors VR.sub.1 to VR.sub.3 are connected in parallel to one another, and one common junction thereof is grounded, whereas the other common junction thereof is connected to the cathode of a power supply 53 whose anode is grounded. Thus, the output laser power is set such that it is maximized at the ground potential.
One ends of the sliding contacts of the first to third variable resistors VR.sub.1 to VR.sub.3 are connected to three-state circuits 55a, 55b and 55c each constructing a line-width reference voltage selector. Outputs of the three-state circuits 55a, 55b and 55c are commonly connected to the other input terminal of the amplifier that constructs the comparator circuit 45.
Laser drive signals LD.sub.0, LD.sub.1 are output from the CPU 48 and fed to a decoder 54. The decoder 54 turns off the laser drive signal when the laser drive signal LD.sub.0 is at "H" (high) level and the laser drive signal LD.sub.1 is at "H" (high) level; the decoder 54 supplies a line type switching signal representative of a fine line drawing to the gate of the three-state circuit 55c when the laser drive signal LD.sub.0 is at "L" (low) level and the laser drive signal LD.sub.1 is at "H" (high) level; the decoder 54 supplies a line type switching signal representative of a middle line drawing to the gate of the three-state circuit 55b when the laser drive signal LD.sub.0 is at "H" (high) level and the laser drive signal LD.sub.1 is at "L" (low) level; and the decoder 54 supplies a line type switching signal representative of a bold line drawing to the gate of the three-state circuit 55a when the laser drive signal LD.sub.0 is at "L" (low) level and the laser drive signal LD.sub.1 is at "L" (low) level. Accordingly, reference voltages set by the resistance values of the variable resistors VR.sub.1 through VR.sub.3 in response to the types of the drawing lines are supplied to the comparator circuit 45 so that the output laser power of the laser diode 30 is made constant.
Potentiometers (X-axis and Y-axis scanners though not shown) or the like of an X-axis scanner driver circuit 50 and a Y-axis scanner driver circuit 51, controlled by the CPU 48 so as to drive the galvanoscanner mirrors 3a, 3b, are varied to output X-and Y-axis line drawing speed voltages V.sub.x and V.sub.y. Then, in response to the control signals from the CPU 48, the X-axis scanner driver circuit 50 and the Y-axis scanner driver circuit 51 drive galvano-motors 3.sub.a 1 and 3.sub.b 1 of the galvanoscanner mirrors 3a, 3b.
In the liquid crystal display apparatus based on the liquid crystal cell 7 actuated by the heat energy from the laser beam, as X-position signal V.sub.x and the Y-position signal V.sub.y supplied to the galvano-motors 3.sub.a 1, 3.sub.b 1 of the galvanoscanner mirrors 3a, 3b from the X-axis and Y-axis scanner driver circuits 50, 51, there are supplied sawtooth-waveform signals 56, 57 relative to the X axis and Y axis within the display area of the liquid crystal cell 7 as shown in FIG. 5.
As described above, in the conventional liquid crystal display apparatus based on the liquid crystal cell 7 actuated by the heat energy from the laser beam, the laser peak power is made constant in response to the line types regardless of the position at which a line is drawn. More specifically, the reference voltage that is determined by the set line width to be drawn is set by the reference voltage setting circuit 52 and supplied to the amplifier 45 forming the comparator circuit as the reference voltage, whereby the constant laser power output is generated from the laser diode 30.
However, when the laser peak power of the laser diode 30 is made constant irrespective of the line drawing position as described above, the following problems occur. That is, when the writing laser beam 6 is not properly focused on the interlayer of the liquid crystal 24 of the liquid crystal cell 7 due to the influence of the optical lens and the optical path difference upon drawing or a temperature at which a line is drawn on the liquid crystal 24 of the liquid crystal cell 7 by the heat energy from the writing laser beam 6 is not uniformly distributed, if the voltage of the variable resistor VR.sub.1 is adjusted such that a line width 58 of a graphic pattern projected onto the display area of the liquid crystal cell 7 or on the screen 9 becomes a predetermined line width, e.g., bold line width at the central portion of the screen 9, then the line width 58 is reduced in width or cannot be drawn at all on the peripheral portion of the screen 9 as shown in FIG. 5.
FIG. 6 of the accompanying drawings shows another example of the conventional laser power density controller circuit. In FIG. 6, like parts corresponding to those of FIGS. 1, 3 and 4 are marked with the same references and therefore need not be described in detail.
As shown in FIG. 6, the laser diode drive data LD.sub.1, LDO are supplied from the CPU 48 to high-order addresses A.sub.1 5, A.sub.1 4 of the ROM (read-only memory) in the vector generator circuit 49 and also supplied to the power controller circuit (comparator circuit) 45. The ROM 49 derives laser-off data, fine line drawing data, middle line drawing data and bold line drawing data. In the laser diode drive data map stored in the ROM 49, an address of bold line, for example, is set to FF.sub.H, an address of middle line is set to BF.sub.H and an address of fine line is set to 7F.sub.H, respectively, whereby the maximum output powers of the laser diode 30 are made constant in response to the types of lines to be drawn.
Further, the X-axis scanning driver circuit 50 and the Y-axis scanning driver circuit 51 which drive the galvanoscanner mirrors 3a, 3b are operated to generate X-axis and Y-axis line drawing speed voltages V.sub.x and V.sub.y by varying the potentiometers thereof or the like. These voltages V.sub.x, V.sub.y are supplied to and converted into digital data by analog-to-digital (A/D) converter circuits 62, 63, respectively. Then, these digital data are supplied to lower-order bit addresses A.sub.0 to A.sub.6 and A.sub.7 to A.sub.1 3 of the ROM 49. Then, the output data X, Y corresponding to the drawing speeds and which are stored in the ROM 49 are supplied to the galvano-motors 3.sub.a 1, 3.sub.b 1 of the galvanoscanner mirrors 3a, 3b or the like, thereby constructing the laser driver unit 60.
In the projection type liquid crystal display apparatus based on the liquid crystal cell 7 that is actuated by the heat energy from the laser beam, as data corresponding to the drawing speed when the bold line drawing data, for example, is selected and which is derived from the ROM 49, there is stored data of value which is shown by a straight line 154 having a predetermined inclination in FIG. 7. In this case, the above drawing speed is a speed which results from synthesizing an X-axis indication speed V.sub.x and a Y-axis indication speed V.sub.y in a vector fashion.
Furthermore, the laser power of the laser diode 30 is controlled by varying pulse widths .tau..sub.1 and .tau..sub.2 of the pulse-width modulated (PWM) waveform in response to the drawing speed as shown in FIGS. 8A and 8B. That is, in the control of the laser power of the laser diode 30, the laser peak power is set and fixed in response to the types of lines to be drawn and the pulse width modulation is carried out in response to the drawing speed.
Bold line, middle line and fine line of predetermined widths are drawn on the liquid crystal surface of the liquid crystal cell 7 in response to the drawing speeds under the condition such that the laser power density of the laser diode 30 is made constant. However, if these lines are drawn on the liquid crystal surface of the liquid crystal cell 7 under the same laser power density in the beginning of and during the drawing, there is then the defect that the line width when the drawing is just started is reduced.
A cause that the line width is unavoidably reduced in the beginning of the drawing of line will be described with reference to FIG. 9.
As shown in FIG. 9 of the accompanying drawings, when bold lines 159a, 159b having a width W and lengths L.sub.1, L.sub.2, for example, are drawn on the liquid crystal cell 7 by the heat energy from the laser beam, the liquid crystal 24 is not heated sufficiently in the beginning of the drawing where a laser beam spot 156 of the laser beam from the laser diode 30 is irradiated on a position shown by a broken line portion 155 so that the liquid crystal 24 is not changed in phase to the isotropic phase. Under this condition, the beam spot 156 is moved at a predetermined drawing speed in the direction shown by an open arrow A in FIG. 9 so that a condition in which a temperature is raised becomes different from that in the beginning of the drawing because a midway portion 158 in which a beam spot 156a that has moved to the next position is moved is pre-heated.
That is, only a hatched portion 157 in the bold line 159b is written. Consequently, the line width in the beginning of the drawing is reduced so that the bold lines 159a, 159b having the predetermined lengths L.sub.1, L.sub.2 cannot be drawn as they are instructed.
Furthermore, as shown in FIG. 3, although the writing laser beam 6 within the laser block 1 contributes to the drawing as the energy for changing the liquid crystal 24 in phase from the smectic A phase into the isotropic phase when it irradiates the liquid crystal 24 disposed within the liquid crystal cell 7, the extra energy is introduced into the photodiode 42 as a reflected-back light 39. The reflected-back beam 39 that has been reflected on the surface of the photodiode 42 is changed into a scattering beam 43. While the scattering beam 43 is partly reflected on the polarizing beam splitter 34, a part of the scattering beam 43 is passed through the polarizing beam splitter 34 and then introduced into the monitor photodiode 36.
In the above-mentioned liquid crystal display apparatus based on a liquid crystal light emitter actuated by the heat from the laser beam, the output laser power of the laser beam from the laser diode 30 relative to the drawing speed of the writing laser beam when the drawing is effected is controlled by the PWM system so that, if the output level of the laser diode 30 is varied, then the line width of the line to be drawn or the like is changed significantly.
For this reason, the output level of the laser diode 30 must be monitored accurately. Therefore, in the optical system shown in FIG. 3, the output level of the laser diode 30 is monitored by the monitor photodiode 36 and the detected output of the photodiode 36 is fed back to the laser diode 30 in an APC fashion,thereby stabilizing the output power of the laser diode 30.
However, if the scattering beam 43 whose intensity is fluctuated is introduced into the monitor photodiode 36 from the position detection photodiode 42, then the output power of the laser diode 30 is fluctuated.